The subject matter disclosed herein relates to coating deposition systems and, more specifically, to micro-channel coating deposition systems for coating articles with micro-cooling channels.
Gas turbine engines may be found in many applications, including industrial turbines, aero-derivative turbines, aircraft turbines, and the like. As an example, in a gas turbine engine for use in an aircraft, air is drawn into the front of the engine, compressed by a shaft-mounted rotary-type compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft and drives the compressor and fan. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
During operation of gas turbine engines, the temperatures of combustion gases may exceed 3,000° F., considerably higher than the melting temperatures of the metal parts of the engine which are in contact with these gases. Operation of these engines at gas temperatures that are above the metal part melting temperatures is a well-established art, and depends in part on supplying a cooling air to the outer surfaces of the metal parts through various methods. Typically, the hot gas path components are cooled by bleeding air from the compressor. The metal parts of these engines that are particularly subject to high temperatures, and thus require particular attention with respect to cooling, are the metal parts forming combustors and parts located aft of the combustor. It should be understood, that while metal parts are the convention at present, looking forward there may be ceramic parts and ceramic matrix composites, for example, that will require similar cooling.
Gas turbine engine cooling art is mature and includes numerous patents for various aspects of cooling circuits and features in the various hot gas path components. For example, in the aviation industry, the combustor typically includes radially outer and inner liners, which require cooling during operation. Industrial turbines more commonly may use can-annular combustor liners or dump combustors. Turbine nozzles include hollow vanes supported between outer and inner bands, which also require cooling. Turbine rotor blades are hollow and typically include cooling circuits therein, with the blades being surrounded by turbine shrouds, which also require cooling. The hot combustion gases are discharged through an exhaust which may also be lined, and suitably cooled.
In all exemplary gas turbine engine components, thin metal walls of high strength superalloy metals are typically used for enhanced durability while minimizing the need for cooling thereof. Various cooling circuits and features are tailored for these individual components in their corresponding environments in the engine. For example, a series of cooling passages, or serpentines, may be formed in a hot gas path component. A cooling fluid may be provided to the serpentines from a plenum, and the cooling fluid may flow through the passages, cooling the hot gas path component substrate and coatings. However, this cooling strategy typically results in comparatively low heat transfer rates and non-uniform component temperature profiles.
Micro-channel surface cooling has the potential to significantly reduce cooling requirements by placing the cooling as close as possible to the heat zone, thus reducing the temperature difference between the hot side and cold side for a given heat transfer rate. Current techniques for forming microchannels typically require specialized techniques, such as, the use of sacrificial fillers, re-entrant grooves, angular deposition techniques, or the like. The use of sacrificial fillers keeps the coating from being deposited within the microchannels while supporting the coating during deposition. Subsequent to deposition of the coating system, the sacrificial filler (fugitive) material is removed. The filling of the channels with a fugitive material, and the later removal of that material presents potential problems for current micro-channel processing techniques. Removal of the sacrificial filler involves potentially damaging processes of leaching etching, or vaporization and typically requires long times. Residual filler material is also a concern. Other micro-channel coating deposition techniques include the fabrication of re-entrant grooves, in which a groove opening at the surface is small enough that the coating particles form a bridge with little or no deposition being deposited inside the groove, and thus within the formed micro-channel. In addition, angular deposition techniques have been utilized for the coating deposition thereby decreasing the line-of-sight into the channel opening. These techniques while accomplishing the deposition of the coating layer may inadvertently allow unwanted coating particles to be deposited into the micro-channels or channel openings.
Additional factors such as the size and shaping of a micro-channel at the coating deposition surface will influence the amount of coating deposited in the micro-channel even though the line-of-sight is present. In part this is due to the increased angle of deposition for any surfaces not normal to the spray direction, for example side walls of the micro-channel.
Accordingly, alternative coating deposition systems and methods would be welcomed in the art.